In general, a spectrum analyzer which analyzes the spectrum of a signal in a wide band continuously sweeps the local frequency of a heterodyne front end to convert each frequency component in an input signal into a frequency component in an intermediate frequency band and detects the level of the signal.
In recent years, with the development of the information society, the amount of information used in various types of communication has increased. Therefore, frequency resources in the microwave band which has been used are insufficient.
Therefore, various types of applications have changed to a high frequency domain and an analysis frequency required for a measurement device (spectrum analysis device) which is used to evaluate the applications has changed from a microwave band to a millimeter-wave band.
In a spectrum analysis device (spectrum analyzer) which is obtainable on the market, a YIG tuned filter (YTF) is used in the front end in order to avoid a measurement error caused by an image signal. The upper limit of the operating frequency of the YTF is about 60 GHz and the YTF is not used to analyze a frequency higher than the upper limit.
When a signal with a frequency (higher than 100 GHz) higher than the upper limit frequency (for example, 60 GHz) which can be measured by a single spectrum analyzer is analyzed, a mixer which is provided outside the spectrum analyzer is generally used to measure the frequency. In general, a harmonic mixer is mainly used as the mixer.
For example, the following Patent Documents 1 and 2 disclose a frequency conversion technique for measuring the frequency higher than the upper limit of the operating frequency of the YTF.